This invention provides new kinds of reinforcing filaments precoated with resin composition for use in a collimated arrangement in a prepreg tape. Boron filaments are the most typical of the new kinds of filaments, though other inorganic filaments currently being developed such as silicon carbide, boron carbide, tungsten carbide, and aluminum oxide filaments have similar features. There are two principal differences between glass filaments, which have been predominant as reinforcement in prepreg tapes, and the new filaments. First, the new filaments are large-diameter monofilaments (generally between 1 or 2 and 10 mils in diameter) whereas glass fibers of reinforcement have consisted of strands, yarns, or rovings of many fine (0.2 - 0.6 mils) monofilaments. Secondly, in comparison to glass filaments the new filaments are quite stiff, boron filaments having a tensile modulus of elasticity of about 60,000,000 pounds per square inch, for example. Because of these differences, the methods used to incorporate glass filaments in prepreg tapes are not effective with the new filaments.
One important inadequacy of the conventional methods arises from a difference in spacing required between filaments in a structural member reinforced with the new large-diameter monofilaments and a member reinforced with conventional glass filaments. A structural member reinforced with the new large-diameter monofilaments contains substantially fewer resin-filled interfilament spaces than a structural member reinforced with fine glass monofilaments, and, as a result, each resin-filled spaced between the new large-diameter monofilaments must accommodate a greater portion of the strain experienced in the member. Furthermore, since the new filaments are much more stiff than glass filaments, they are displaced from their normal position in the structural member a lesser amount than glass filaments are displaced under the same stress, and the resin composition between filaments must undergo more strain to make up the difference. The result of these larger strains between adjacent large-diameter monofilaments is that larger resin-filled spaces are required between the new monofilaments than between fine glass monofilaments, both in prepreg tape and ultimately in the reinforced member made from the tape.
Although sufficient resin is inherently impregnated between fine glass monofilaments when a web of roving is appropriately passed through a dip tank to make prepreg tape, the needed spacing between the now monofilaments cannot be obtained so easily. Large numbers of the new monofilaments, which are quite brittle, stiff, and difficult-to-handle, must be collimated in a compact monolayer, and uniform, controlled spacing must be provided between the monofilaments. Then, while held in the collimated, spaced relation, the monofilaments must be integrated into tape with a matrix of resin. More than that, the uniform spacing between monofilaments must be maintained in the reinforced structural member, despite the fact that it is necessary that the resin laid-up layers of tape flow under the heat and pressure of the molding operation. In conventional prepreg tapes this needed flow would "wash" the filaments together.
A different obstacle is that the hot-melt and solvent-coating methods used to make glass-filament prepreg tape are inadequate with the new, large-diameter, high-modulus monofilaments. Resin compositions having enhanced strain capability when molded are especially desirable in tapes of this invention to provide tough structural members adapted to accommodate the previously described large interfilament strains. However, these tough resin compositions tend to be of high viscosity and are thus difficult to coat by hot-melt methods.
Solvent-coating methods are undesirable with the new high-modulus monofilaments because of the problems raised by the brittleness of these monofilaments. During the time that breaks are repaired, the resin composition in the portion of tape in the solvent-removing oven is likely to cure excessively.